U.S. patent application number 11/568605 was filed with the patent office on 2008-09-11 for robot-controlled optical measurement array, and method and auxiliary mechanism for calibrating said measurement array.
This patent application is currently assigned to KUKA Roboter GmbH. Invention is credited to Thomas Ibach, Bernhard Laubel, Matej Leskovar, Holger Linnenbaum, Martin Paskuda.
Application Number | 20080216552 11/568605 |
Document ID | / |
Family ID | 34967769 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080216552 |
Kind Code |
A1 |
Ibach; Thomas ; et
al. |
September 11, 2008 |
Robot-Controlled Optical Measurement Array, and Method and
Auxiliary Mechanism for Calibrating Said Measurement Array
Abstract
Disclosed is a robot-controlled optical measurement array (1)
comprising an optical sensor (2) that is fastened to a spacer (3).
Reference marks (22) are provided on the spacer (3) and/or on a
sensor (2) housing (2'). Said optical measurement array (1) is
calibrated by means of an auxiliary device (13) that is placed on
the optical measurement array (1) and is provided with a sensor
target (16) which is disposed on the auxiliary device so as to lie
within one measurement space (17) of the optical sensor (2) when
the optical measurement array (1) and the auxiliary device (13) are
in the assembled state. In order to calibrate the optical
measurement array (1), measured values of the sensor target (16)
are generated with the aid of the sensors (2), said measured values
being used for calculating the three-dimensional position of the
sensor coordinate system (10) in relation to the sensor target
(16). Furthermore, measurements of the three-dimensional positions
of the reference marks (22) and the auxiliary device are taken
using an additional (stationary) measuring apparatus (21). The
three-dimensional position of the sensor coordinate system (10)
relative to the reference marks (22) can be determined with great
accuracy from a combination of all of said measurements.
Inventors: |
Ibach; Thomas; (Rastatt,
DE) ; Laubel; Bernhard; (Bischweier, DE) ;
Leskovar; Matej; (Munchen, DE) ; Linnenbaum;
Holger; (Rastatt, DE) ; Paskuda; Martin;
(Steinmauern, DE) |
Correspondence
Address: |
WOOD, HERRON & EVANS, LLP
2700 CAREW TOWER, 441 VINE STREET
CINCINNATI
OH
45202
US
|
Assignee: |
KUKA Roboter GmbH
Augsburg
DE
|
Family ID: |
34967769 |
Appl. No.: |
11/568605 |
Filed: |
May 3, 2005 |
PCT Filed: |
May 3, 2005 |
PCT NO: |
PCT/EP2005/004796 |
371 Date: |
August 17, 2007 |
Current U.S.
Class: |
73/1.01 |
Current CPC
Class: |
G01B 11/002 20130101;
B25J 9/1692 20130101; G01B 5/0025 20130101; G01B 11/2518
20130101 |
Class at
Publication: |
73/1.01 |
International
Class: |
G12B 13/00 20060101
G12B013/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 4, 2004 |
DE |
10 2004 021 892.7 |
Claims
1. An optical measurement array (1) for use on a multiaxis
manipulator, particularly an industrial robot (6), with a spacer
(3) that can be mounted on the manipulator (6) and on which an
optical sensor (2) is fixed, wherein reference marks (22) are
provided on the spacer (3) and/or on a housing (2') of the sensor
(2).
2. The optical measurement array according to claims 2 and 3,
characterized by the fact that the reference marks (22) consist of
retroreflecting spheres (27).
3. The optical measurement array according to one of claims 1-4,
characterized by the fact that the reference marks (22) can be
connected to the spacer (3) and/or the housing (2') of the sensor
(2) such that they can be positioned in a removable and
positionally accurate fashion.
4. An auxiliary device (13) for calibrating an optical measurement
array (1) that can be mounted on a manipulator (6), characterized
by the fact that the auxiliary device (13) features a mounting
region (14) for fixing the auxiliary device (13) relative to the
optical measurement array (1), as well as a sensor target (16) that
is arranged on the auxiliary device 13 such that it lies in a
measurement space (17) of the optical sensor (2) in be assembled
state of the auxiliary device 13 and the optical measurement array
(1).
5. The auxiliary device according to claim 4, characterized by the
fact that the auxiliary device is provided with reference marks
(22'), particularly with retroreflecting elements (27').
6. A method for calibrating an optical measurement array (1) that
can be mounted on a manipulator (6) and on which reference marks
(22) are provided, wherein said method comprises the following
steps: an auxiliary device (13) according to claim 4 or 5 is
mounted on the optical measurement array (1), measured values of
the sensor target (16) of the auxiliary device (13) are generated
with the aid of the sensor (2), the position in space of the
reference marks (22) and the position in space of the auxiliary
device (13) are determined with the aid of an additional measuring
device (21), the position of a sensor coordinate system (10)
relative to the reference marks (22) is determined from a
combination of the measurements of the additional measuring device
(21) and the measurements of the sensor (2).
7. The method according to claim 6, characterized by the fact that
the additional measuring device consists of a laser tracker
(28).
8. The method according to claim 6 or 7, characterized by the fact
that the optical measurement array (1) is mounted on the
manipulator (6) while the calibration process is being carried out.
Description
[0001] The invention pertains to an optical measurement array for
use on a measuring robot. The invention furthermore pertains to an
auxiliary device and a method for calibrating the optical
measurement array.
[0002] A robot-controlled optical sensor for measuring one or more
measurement areas of workpieces in the production environment, for
example, on car bodies, is known from U.S. Pat. No. 6,321,137 B1.
The optical sensor is mounted on the robot arm and is positioned
relative to the workpiece in selected measurement areas with the
aid of the robot. The utilization of an optical sensor provides the
advantage that such a sensor measures in a contactless fashion and
therefore has a substantially faster measuring speed and a lower
sensitivity to vibrations than a tactile sensor. This makes it
possible to carry out fast and robust measurements in the
production environment. The positioning of the optical sensor with
the aid of a robot provides the additional advantage of a high
flexibility and reduced costs; the positioning with the aid of a
robot also makes it possible to achieve an adequate reproducibility
and accuracy of the measurement results.
[0003] However, this can only be realized if a highly accurate
calibration of the complete system--consisting of the robot and the
optical sensor--is carried out prior to the actual measuring
operation. In this case, it is common practice to initially
calibrate the robot by determining its axial error and compensating
this axial error with the aid of the control system. In the
calibration of the optical sensor system, the error of the sensor
optics is compensated and the position of the sensor coordinate
system relative to an external reference point, for example, the
sensor housing, is determined. It is also necessary to relate the
position of the sensor coordinate system to the position of the
robot coordinate system in order to determine the position of
measuring points of the sensor in the robot coordinate system.
[0004] In this context, U.S. Pat. No. 6,321,137 B1 proposes to move
the optical sensor into different positions in space relative to a
reference body with the aid of the robot and to carry out a
calibration of the complete system based on the data from sensor
measurement of the reference body obtained in these spatial
positions. In this case, however, only a relatively low accuracy is
empirically achieved that does not suffice for many applications in
the production environment, particularly for measurements on car
bodies (shells).
[0005] Consequently, the invention is based on the objective of
designing a robot-controlled optical measurement array in such a
way that it can be quickly and easily calibrated. The invention
furthermore aims to propose a method and an auxiliary device that
make it possible to carry out this calibration process quickly and
in a reproducible fashion.
[0006] According to the invention, these objectives are attained
with the characteristics of claims 1, 4 and 6.
[0007] According to these characteristics, the optical measurement
array comprises an optical sensor that is mounted on a robot with
the aid of a spacer, wherein reference marks are provided on the
spacer and/or the sensor housing. These reference marks consist,
for example, of steel spheres, the centers of which can be
determined in a highly accurate fashion with the aid of a tactile
measuring means, particularly the sensing element of a coordinate
measuring machine. Alternatively, the reference marks can also
consist of retroreflectors, in which case a highly accurate
measurement of the position of these reference marks can be
realized with the aid of a laser tracker. According to one
particularly advantageous embodiment, the reference marks consist
of retroreflector spheres such that the position of these reference
marks can be measured in a tactile as well as an optical fashion.
These spheres are preferably mounted on the optical measurement
array in a detachable fashion, namely such that they can be removed
(during the measuring operation) and reattached to the optical
measurement array in a precisely reproducible fashion in order to
carry out a calibration or a validation measurement.
[0008] The calibration of the optical sensor is realized with an
auxiliary device that can be mounted on the optical measurement
array--preferably in a precisely reproducible fashion. The
auxiliary device features a sensor target that is arranged to the
auxiliary device such that it lies within a measurement space of
the optical sensor in the assembled state of the auxiliary device
and the optical measurement array.
[0009] In the calibration of the optical measurement array, the
position of the sensor coordinate system (also referred to as
TCP=Tool Center Point) relative to the reference marks fixed on the
measurement array needs to be determined. The auxiliary device is
fixed on the optical measurement array for this purpose. One or
more measurements of the sensor target of the auxiliary device are
then carried out with the aid of the optical sensor; the position
of the sensor target in the sensor coordinate system is determined
based on the measurement(s). In addition, the position in space of
the reference marks fixed on the optical measurement array relative
to the auxiliary device is measured by means of another (optical or
tactile) measuring system. The position of the sensor coordinate
system of the optical measurement array relative to the reference
marks is calculated from a joint evaluation of these measurements.
In order to determine the position in space of the auxiliary device
relative to the reference marks of the optical measurement array in
a highly accurate fashion, it is advantageous also to provide the
auxiliary device with reference marks.
[0010] A suitable arrangement of the reference marks on the optical
measurement array and on the auxiliary device makes it possible to
determine the position in space of the sensor coordinate system
relative to the reference marks of the optical measurement array in
a highly accurate fashion. This calibration of the optical
measurement array can be carried out in a measurement space, for
example, in a (tactile) coordinate measuring device. Since a small,
lightweight and robust auxiliary device that can be easily handled
is used for the calibration process, the calibration of a robot, on
the robot arm of which the optical measurement array is mounted,
can also be carried out directly in the factory environment. In
this case, the reference marks fixed on the optical measurement
array can also be used for determining the position in space of the
optical measurement array relative to the robot coordinate system
in a highly accurate fashion--for example, with the aid of a laser
tracker--and in this fashion to detect and compensate for the
errors and inaccuracies occurring during the transformation of the
sensor measured values into the robot coordinate system.
[0011] The invention is described in greater detail below with
reference to an embodiment that is illustrated in the figures.
[0012] The figures show:
[0013] FIG. 1, a schematic representation of a robot-controlled
optical measurement array, and
[0014] FIG. 2, a schematic representation of an inventive auxiliary
device for calibrating the optical measurement array according to
FIG. 1.
[0015] FIG. 1 shows a schematic representation of an inventive
optical measurement array 1 with an optical sensor 2 that is
mounted on a spacer 3. The spacer 3 features a flange 4 that serves
for mounting the optical measurement array 1 on a robot arm 5 of a
multiaxis manipulator, particularly a six-axis industrial robot 6.
An evaluation unit 7 is provided for storing and evaluating the
measuring data of the optical sensors 2. The robot 6 is connected
to a robot control unit 8 in order to control the motion of the
robot arm 5. The robot 6 is calibrated prior to the measuring
operation by determining and compensating for its axial error with
the aid of the control unit 8.
[0016] In the measuring operation, measured values of an object 9
to be measured are obtained with the aid of the optical sensors 2,
with these measured values being generated in a sensor coordinate
system 10 (that is moved together with the optical measurement
array 1). In order to transform the measured values into a
stationary robot coordinate system 11 or a coordinate system 12 of
the object 9 to be measured, the position in space of the sensor
coordinate system 10 in the robot coordinate system 11 as a
function of the movement of the robot arm 5 needs to be known. This
requires a "calibration" the optical measurement array 1.
[0017] The auxiliary device 13 shown in FIG. 2 is used for this
purpose. The auxiliary device is realized in the form of
"spectacles" that are attached to the optical measurement array 1
for the calibration process and consequently feature a mounting
region 14 for fixing the auxiliary device on the housing 2' of the
sensor 2. In order to ensure that the auxiliary device can be fixed
relative to the sensor 2 in a reproducible fashion, the sensor
housing 2' is advantageously provided with limit stops 15 that
define the relative position of the mounting region 14 with respect
to the sensor housing 2'. A sensor target 16 is arranged on the
auxiliary device 13 such that it is situated, when the auxiliary
device 13 is attached to the sensor 2, in the measurement volume 17
of the optical sensor 2. The sensor target 16 features geometric
characteristics 18, 19 that make it possible to quickly and
accurately calculate the position and the orientation of the sensor
2 relative to the sensor target 16. These geometric characteristics
18, 19 are adapted to the measuring principle of the optical
sensors 2. The optical sensor 2 used in the described embodiment is
able to generate three-dimensional measuring points (with the aid
of the laser stripe method), as well as to detect two-dimensional
characteristics (with the aid of a gray scale image evaluation). In
this case, the sensor target 16 comprises--as indicated in FIG.
2--one or more vertical steps 18 (the position in space of which
can be measured with the aid of the light section method). The
sensor target 16 furthermore features several bores 19 (the centers
of which can be calculated by means of image processing of the gray
scale images). This makes it possible to determine in a highly
accurate fashion the position in space and the angular position of
the sensor target 16 in the sensor coordinate system 10.
[0018] In order to determine the position in space of the auxiliary
device 13 during the course of calibration of the optical
measurement array 1 with the aid of a stationary measuring device
21, for example, a laser tracker 28, the auxiliary device 13 needs
to be provided with reference marks 22' that can be measured by the
stationary measuring device 21. These reference marks 22' may
consist, for example, of retroreflector spheres 27', the positions
of which relative to the sensor target 16 are determined in a
calibration process (carried out prior to the calibration of the
optical measurement array 1), namely with the aid of a tactile
coordinate measuring machine. Alternatively, these reference marks
22' may also consist of bores 19 of the sensor target 16 that are
realized in the form of through-holes such that the position of the
bores 19 can be determined by carrying out a measurement with the
stationary measuring device 21 and deducing the position of the
sensor target 16 based on this measurement (provided that the local
wall thickness 29 of the auxiliary device 13 and the step height 18
of the sensor target 16 are known).
[0019] The optical measurement array 1 is calibrated by attaching
the auxiliary device 13 to the optical measurement array 2 (which
is mounted on the robot arm 5). Three measurements are carried out
in this state:
[0020] 1. Measurements of the sensor target 16 are carried out with
the aid of the sensor 2, with the position in space of the sensor
coordinate system 10 relative to the sensor target 16 being
determined based on these measurements.
[0021] 2. Measurements of the reference marks 22' of the auxiliary
device 13 are carried out with the aid of the stationary measuring
device 21, with the position in space of the auxiliary device 13
(and consequently also the position of the sensor target 16 that is
rigidly connected to the auxiliary device 13) being calculated in a
coordinate system 20 of the stationary measuring device 21 based on
these measurements.
[0022] 3. The position in space of the optical measurement array 1
in the coordinate system 20 of the stationary measuring device 21
is determined with the aid of the stationary measuring device 21.
For this purpose, reference marks 22 are provided on the optical
measurement array 1 such that they can be measured with the
stationary measuring device 21. In the described embodiment, in
which the stationary measuring device 21 consists of a laser
tracker, these reference marks 22 consist of retroreflector spheres
27 that are respectively mounted on the spacer 3 and the housing 2'
of the optical sensor 2. In order to mount the retroreflector
spheres 27, the spacer 3 and the sensor housing 2' contain threaded
holes 24, into which so-called "nesting elements" 25 are screwed.
These "nesting elements" 25 are realized in the form of annular
plates, into which the retroreflector spheres 27 can be
reproducibly inserted in an accurately defined position. The
"nesting elements" 25 are provided with magnetic elements such that
the retroreflector spheres 27 are securely fixed and held in the
"nesting elements" 25--regardless of the spatial orientation of the
"nesting elements" 25. In the embodiment shown in FIG. 2, only a
few threaded holes 24 are fitted with "nesting elements" 25, and
only a few of these "nesting elements" 25 are illustrated with
retroreflector spheres 27 fixed therein. A person skilled in the
art is familiar with the fact that the number and position of the
reference marks 22 significantly influence the accuracy in
determining the position in space of the optical measurement array
relative to the coordinate system 20 of the stationary measuring
device 21. Careful selection of the reference marks 22 is therefore
crucial to the quality of the calibration result.
[0023] The position of the sensor coordinate system 10 relative to
the reference marks 22 fixed on the optical measurement array 1 can
be determined in a highly accurate fashion from a combination of
measurements 1., 2. and 3. If the optical measurement array 1 is
moved into different positions in space with the aid of the robot
6, the current position of the sensor coordinate system 10 can be
deduced for each of these positions in space based on a measurement
of the reference marks 22. If the stationary measuring device 21
were calibrated to the coordinate system 12 of the object 9 to be
measured, the current position of the sensor coordinate system 10
(and therefore also the position in space of the measuring data
obtained by means of the sensor 2) can be calculated in the
coordinate system 12 of the object 9 to be measured (e.g., in a
vehicle coordinate system). It is therefore expedient, before the
beginning of the actual measuring operation, to measure with the
aid of the stationary measuring device 21 and the reference marks
22 the position in space of the optical measurement array 1. This
is determined for all spatial positions in which measurements are
to be carried out on the object 9 to be measured during the
measuring operation.
[0024] The support of the retroreflector spheres 27 in magnetic
"nesting elements" 25 provides the advantage that the
retroreflector spheres 27 can be removed during the measuring
operation; this reduces the weight of the optical measurement array
1 and therefore the load on the robot arm 5 during the measuring
operation; the space requirement of the optical measurement array 1
is also reduced. If it becomes necessary to carry out another
calibration of the optical measurement array 1 or of the measuring
robot 6 during the measuring operation (e.g., due to a crash of the
measuring robot 6), the retroreflector spheres 27 can be inserted
into the "nesting elements" 25 at any time in a reproducible
fashion.
[0025] With respect to the above-described design of the reference
marks 22 in the form of retroreflector spheres 27, it should be
noted that the reference marks 22 may also be realized in any other
suitable fashion. The measurement of the reference marks 22 carried
out with the aid of the laser tracker 28 during the calibration
process can also be replaced with any other optical or tactile
measurement method.
[0026] The auxiliary device 13 can be used in connection with
optical measurement arrays 1 that are based on different
measurement principles, particularly for laser stripe sensors, CCD
cameras with gray scale image processing, strip projection sensors,
etc.
* * * * *